The present invention relates to a process for purifying amino acid-containing solutions by electrodialysis, an amino acid-containing solution being employed which is obtained from a fermentation.
Amino acids can be produced from a carbon source such as, for example, molasses and raw sugar by means of fermentation. The solutions generally accumulating in the fermentation contain in addition to about 10% by weight of amino acid large amounts of contaminants such as, for example, also biomass.
According to the processes used up until now in accordance with the prior art numerous process steps are necessary for purifying this solution obtained from a fermentation in order to produce the amino acid in the commonly marketed form, for example as lysine hydrochloride. Usually, large amounts of salts such as, for example, ammonium sulfate accumulate, approximately 1 kg of ammonium sulfate per kg of amino acid being generated as a product of value. Additionally, large amounts of wastewater accumulate and have to be worked up and disposed of. A preferably commercially used process for working up discharges from amino acid fermentation is essentially based on ion exchange treatment. The discharges from the fermentation are mixed with acid and then loaded onto a cation exchange column or an anion exchange column. The amino acid in its ionic form displaces the corresponding ions of the cation or anion exchange column and thus is retained by the column. The biomass can be filtered off before or after the ion exchange treatment. The mother liquor obtained after passage still contains approximately 0.1% by weight of amino acid and a load of about 1 metric ton of salt, for example ammonium sulfate, per metric ton of amino acid. This solution can be disposed of by dumping at sea which is objectionable, in particular ecologically, owing to the high salt load of the solution to be dumped at sea.
As an alternative to this, the mother liquor which contains essentially ammonium sulfate and biomass may be worked up in numerous evaporation and filtration stages, it being possible for ammonium sulfate to be retrieved. However, the accumulating mother liquor has still to be disposed of. Moreover, this procedure is very labor-intensive and has a high energy consumption.
The amino acid retained on the ion exchange resin is eluted, for example, with aqueous ammonia. Ammonia is stripped off the accumulating eluate, large amounts of aqueous ammonia having to be evaporated. The eluted amino acid is then mixed with acid, for example hydrochloric acid, crystallized and dried. The amino acid obtained in this way is  greater than 98.5% pure and is called xe2x80x9cfeed-gradexe2x80x9d amino acid. In order to obtain even less contaminated, so-called xe2x80x9cfood-gradexe2x80x9d or xe2x80x9cpharma-gradexe2x80x9d amino acid, feed-grade product has to be subjected to a further purification procedure, for example active carbon treatment and/or repeated crystallization. Further details with respect to the purification processes in accordance with the prior art described above can be found in the PERP report 5357. The specifications food-grade and pharma-grade used here refer to the relevant purity requirements according to the US FDA, where the appropriate standard for lysine HCl is to be understood correspondingly or analogously. According to this, the term xe2x80x9cwith high purityxe2x80x9d used here also refers to the food-grade or pharma-grade standard for lysine according to the US FDA and is in this context also used either correspondingly or analogously.
Electrodialysis is a very elegant purification or separation process which can fractionate, inter alia, mixtures of amino acids. A process of this type is described, for example, by Sandeaux et al. in J. Chem. Technol. Biotechnol. 71 (1998), pp. 267 ff. According to this scientific publication, protein hydrolysates obtained by acidic hydrolysis of animal residues or human residues such as, for example, feathers or hairs are initially neutralized and decolorized by activated carbon and then subjected to a multistage electrodialysis, various fractions of the individual amino acids being obtained.
WO 91/02584 describes a process for removing amphoteric compounds from aqueous solutions by means of 3-cycle electrodialysis with bipolar membranes.
U.S. Pat. No. 5,002,881 describes a fermentation process for producing lactic acid by a combination of microfiltration, reverse osmosis and electrodialysis. U.S. Pat. No. 4,882,277 discloses a further process for removing organic acids from fermentation broths by means of anion exchange membranes and producing the free acid by bipolar membranes.
The above summary of the prior art reveals that the purification of amino acids from discharges from fermentations has up until now merely been carried out by means of ecologically disadvantageous and/or costly processes.
It was an object of the present invention to provide an improved process for purifying amino acid from discharges from fermentations. This object was achieved by applying electrodialysis to this problem.
Accordingly, the present invention relates to a process for purifying amino acid-containing solutions by electrodialysis (d), an amino acid-containing solution being employed which is obtained from a fermentation for producing at least one amino acid.
Using this process it is possible to avoid numerous process steps which are indispensable according to the processes of the prior art. Large amounts of ammonium sulfate or other extraneous salts are not generated inevitably in this case. Electrodialysis carried out in a suitable way makes it further possible to produce amino acid as free base or free acid. It is further possible to obtain high amino acid yields of preferably  greater than 90%, further preferably  greater than 95%, based on the employed amount of amino acid at high current densities and therefore correspondingly high capacities of the electrodialysis. Accordingly, the present invention also relates to the use of electrodialysis for purifying amino acid-containing solutions obtained from the fermentation up to an amino acid content of  greater than 98.5% (% by weight).
As stated above, the process according to the invention relates to the purification of amino acid-containing solutions which are obtained from the fermentation. It is possible to employ here generally all amino acid-containing solutions obtainable by the various fermentation processes. Further stages such as, for example, ion exchange treatment and/or crystallization may be carried out between the actual fermentation process and the electrodialysis, merely the resulting mother liquor then being treated by means of electrodialysis.
Therefore the present invention also relates to a process comprising the following additional stages (a) to (c):
(a) treatment by means of an ion exchanger of an amino acid-containing solution obtained from the fermentation for producing at least one amino acid,
(b) crystallization of an amino acid-containing solution obtained in (a), where an amino acid-containing mother liquor is obtained, and
(c) introduction of the amino acid-containing mother liquor obtained in (b) into an electrodialysis apparatus.
Besides the mentioned process stages, further stages such as, for example, filtration before stage (a) may be carried out.